For example, a developing device which develops an electrostatic latent image on a photoreceptor surface into a visible form using a two-component developer (hereinafter just referred to as the developer) including carrier and toner is often used in dry-type copying machines. In such a developing device, the toner is consumed during a developing operation, but the carrier is not consumed and remains in the developing device. Thus, the carrier deteriorates as the frequency of stirring the carrier and the toner in the developing device increases. Consequently, a resin coating layer on the surface of the carrier is removed and the toner adheres to the surface thereof. As a result, the charging property of the developer gradually deteriorates.
In order to solve such a problem, for example, Japanese Publication for Examined Patent Application No. 21591/1990 discloses a device which restrains the deterioration of the charging property by continuously supplying a small amount of carrier to the developing device as well as toner that is consumed during a developing operation. In this device, when the developer is increased excessively in a developer container by the supply of the carrier, it overflows and is discharged through a discharge opening in a wall of the developer container. The discharged developer is collected in a developer collecting container. By successively suppling the toner and carrier and discharging the developer in a repeated manner, the deteriorated developer in the developer container is replaced with newly supplied toner and carrier. With this structure, the charging property of the developer is maintained and the deterioration of the image quality of a copy is restrained.
However, since the deteriorated developer in the developer container overflows or is discharged through the discharge opening in the wall of the developer container, a desired replacement ratio of deteriorated developer and newly supplied developer is not maintained. As a result, the charging property of the developer varies and the image quality of a copy deteriorates.
For example, when a plurality of copies of a high density document are repeatedly produced, the toner consumption is excessively increased and the amount of developer in the developer container is decreased. However, even if the carrier is provided in this state, the developer does not overflow as it does not reach the discharge opening. Namely, the deteriorated developer can never be discharged.
On the other hand, if the carrier is supplied when a large amount of developer is stored in the developing device, the newly supplied carrier is likely to overflow instantly. Thus, the replacement of the deteriorated carrier and the newly supplied carrier is not carried out in a desired manner.
As described above, with this structure it is difficult to supply the carrier and discharge the developer in a desired manner, and the deterioration of the image quality of a copy due to the decreased charging property of the developer is not satisfactorily compensated.
Moreover, in the latter case, an excessive amount of carrier needs to be supplied in order to compensate for an undesired discharge caused when the newly supplied carrier overflows, resulting in an increase in the consumption of developer.
Furthermore, for example, if a copying machine incorporating the above-mentioned developing device is moved, a large amount of developer tends to be discharged through the discharge opening as the copying machine is tilted or receives impact by the movement. Thus, the developer is wastefully discharged, and the charging property of the developer is considerably varied due to the excessive reduction in the amount of developer in the developer container, resulting in a considerable change in the image quality of a copy.
U.S. Pat. No. 5,095,338 discloses a developing device including a permanent magnet mounted in the vicinity of the discharge opening of the developer container. The magnetic field of the permanent magnet keeps the developer from flowing down from the discharge opening. A current is caused to flow through an electromagnet disposed adjacent to the permanent magnet until a detector which is mounted on an upper position of the side wall of the developer container senses that the developer in the developer container comes below a predetermined height. Consequently, the magnetic field produced by the permanent magnet is weakened by the magnetic field of the electromagnet, and the developer is automatically discharged through the discharge opening.
With this structure, however, since the supplying of new developer to the developer container and the discharging of the developer from the developer container are performed independently, it is necessary to control the supplying of the developer and the discharging of the developer separately. It is thus difficult to accurately control the replacement ratio of the old and new developer in a simplified manner. Accordingly, it is difficult to keep a charging property of the developer and to maintain favorable image quality of a copy.